1. Field of the Invention
The present invention is directed to scanning probe microscopes (SPMs), and, more particularly, relates to a scanner for a SPM that can acquire high-quality images at high acquisition rates and to a method of operating such a scanner.
2. Description of Related Art
Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are devices which typically use a tip and low forces to characterize the surface of a sample down to atomic dimensions. Generally, SPMs include a probe having a tip that is introduced to a surface of a sample to detect changes in the characteristics of a sample. By providing relative scanning movement between the tip and the sample, surface characteristic data can be acquired over a particular region of the sample and a corresponding map of the sample can be generated.
The atomic force microscope (AFM) is a very popular type of SPM. The probe of the typical AFM includes a very small cantilever which is fixed to a support at its base and which typically has a sharp probe tip attached to the opposite, free end. The probe tip is brought very near to or into contact with a surface of a sample to be examined, and the deflection of the cantilever in response to the probe tip's interaction with the sample is measured with an extremely sensitive deflection detector, often an optical lever system such as described in Hansma et al. U.S. Pat. No. RE 34,489, or some other deflection detector such as strain gauges, capacitance sensors, etc. The probe is scanned over a surface using a high resolution three-axis scanner acting on the sample support and/or the probe. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography or some other surface property of the sample as described, e.g., in Hansma et al. U.S. Pat. No. RE 34,489; Elings et al. U.S. Pat. No. 5,266,801; and Elings et al. U.S. Pat. No. 5,412,980.
AFMs may be designed to operate in a variety of modes, including contact mode and oscillating mode. This is accomplished by moving either the sample or the probe assembly up and down relatively perpendicular to the surface of the sample in response to a deflection of the cantilever of the probe assembly as it is scanned across the surface. Scanning typically occurs in an “x-y” plane that is at least generally parallel to the surface of the sample, and the vertical movement occurs in the “z” direction that is perpendicular to the x-y plane. (Note that many samples have roughness, curvature and tilt that deviate from a flat plane, hence the use of the term “generally parallel.”) In this way, the data associated with this vertical motion can be stored and then used to construct an image of the sample surface corresponding to the sample characteristic being measured, e.g., surface topography. Similarly, in another preferred mode of AFM operation, known as TappingMode™ (TappingMode™ is a trademark owned by the present assignee), the tip is oscillated at or near a resonant frequency of the associated cantilever of the probe. A feedback loop attempts to keep the amplitude of this oscillation constant to minimize the “tracking force,” i.e. the force resulting from tip/sample interaction. (Alternative feedback arrangements keep the phase or oscillation frequency constant.) As in contact mode, these feedback signals are then collected, stored and used as data to characterize the sample. Note that “SPM” and the acronyms for the specific types of SPMs may be used herein to refer to either the microscope apparatus or the associated technique, e.g., “atomic force microscopy.”
Regardless of their mode of operation, AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum by using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers fabricated using photolithographic techniques. Because of their resolution and versatility, AFMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research.
As with most measuring devices, AFMs often require a trade off between resolution and acquisition speed. That is, some currently available AFMs can scan a simple surface with sub-angstrom resolution. These scanners are capable of scanning only relatively small sample areas, and even then, at only relatively low scan rates. Traditional commercial AFMs usually require a total scan time typically taking several minutes to cover an area of several microns at high resolution (e.g. 512×512 pixels) and low tracking force. The practical limit of AFM scan speed is a result of the maximum speed at which the AFM can be scanned while maintaining a tracking force that is low enough not to damage or cause minimal damage to the tip and/or sample. Professor Toshio Ando at Kanazawa University in Japan has made tremendous progress with high-speed AFM using an AFM that scans mm-sized samples over small distances, typically less than 2 um. Professor Ando has achieved video scan rates with high resolution for this combination of small samples and small scan sizes.
Other systems, typically called “tip scanners,” are known or have been proposed and/or implemented in which the probe is mounted on the scanner. One such system is incorporated in a line of instruments marketed by Veeco Instruments under the name Dimension®. That system employs a relatively massive tube scanner for the Z-actuator and has relatively low bandwidth. Another system is disclosed in Published U.S. Application Ser. No. 2006/00272398 to Hwang. In the system of the Hwang application, the probe is mounted on an actuator that, in turn, is mounted on an optical objective that focuses incoming laser light. The objective, in turn, is mounted on an x-y actuator. However, because the objective and other optics of the system are fixed relative to the probe, relatively large probes (having a width of at least of 20 μm, a length of more than 40 μm) are required to assure positioning of the focused laser beam on the cantilever. The typical probes used also have a resonant frequency Fo of roughly 400 kHz and a quality factor Q of around 400. The resulting response bandwidth for these probes is of the order of Fo/Q≈1 kHz. Due in part to its low-bandwidth probe, the resulting system has a maximum scan rate of less than 30 Hz (or 30 scan lines per second), and more typical imaging speeds are around 1 Hz.
On the other hand, SPMs that can acquire data rapidly can also suffer unacceptable tradeoffs. One such system is marketed by Infinetisma under the name Video AFM™. The Video AFM operates at video rates but with significant compromises to signal-to-noise ratio and resulting image quality. The Infinitesima system also operates in contact mode with force feedback that is not fast enough to respond to variations in sample corrugation within a scan line. In this system, the sample or the probe is mounted on a tuning fork. The probe is driven into contact with the sample while the sample or the probe is scanned by vibrating the tuning fork at or near its resonant frequency. Because the tuning forks need to be quite small (typically on the order of a few mm in size) to achieve high resonant frequencies, they are very sensitive to being loaded by extra mass. As a result, only very small (on the order of a few mm in size) samples or cantilever substrates can be mounted to the tuning fork without degrading the performance.
It is known to combine an AFM with a conventional optical microscope to provide a view of the surface features of the sample. Notably, high performance microscope objectives have a short working distance and must be positioned close to the sample surface. High resolution optical imaging is therefore difficult to implement in combination with traditional AFM detectors because there is insufficient space between the bottom of the objective and the probe to accommodate the geometry for the incoming and outgoing detection beams. Because of the weight of the optical microscope, it is difficult to incorporate the optics of an optical microscope into the scanner of the AFM without unacceptably reducing the instrument's scan rate.
Some optical microscope-equipped SPMs have attempted to overcome this limitation by directing laser light through the microscope objective. One such system has been commercialized by Surface Imaging Systems under the name ULTRAOBJECTIVE™ and is disclosed in international publication number WO 01/23 939. In the ULTRAOBJECTIVE™ system, a near field AFM probe, a z actuator assembly for the probe and optical focusing system are provided in a single housing in order to provide an interchangeable objective that can be inserted in the objective turret of an optical microscope. Its objective is fixed relative to the probe, and it lacks any mechanism for dynamically focusing the laser beam onto the probe.
Another drawback of conventional optical microscope equipped AFMs is that the optical microscope is provided only to allow the user to inspect the sample. It plays no role in focusing the laser beam on the cantilever. Hence, even if the system were provided for focusing the light spot on the cantilever, no mechanism would be available to provide the user with optical feedback during a focusing process.
The need therefore has arisen to provide a tip scanner for a SPM of a sufficiently high lowest fundamental lowest resonant frequency and sufficiently high resolution to make high resolution scans at high scan rates.
The need has additionally arisen to provide a SPM that can focus and target a sensing light beam on a probe, thereby permitting the use of relatively small focused beam spots and small probes in the SPM.